PROJECT SUMMARY ABSTRACT Periodontitis is the second most prevalent infectious disease of mankind. It is a chronic inflammatory condition that leads to the destruction of tooth-supporting structures, including the gingiva, periodontal ligament and alveolar bone. It is the main cause of tooth loss and induces systemic implications, including cardiovascular disorders, preterm birth and cancer. While a microbial biofilm is the main cause of periodontitis, it is increasingly recognized that the host inflammatory responses determine the outcome of periodontal disease. Therefore elucidation the molecular signaling network controlling inflammatory responses is crucial for understanding and treating this disease. Dysregulation of signaling networks that serve to control inflammation is often causative for periodontitis. Toll-like receptor (TLR) signaling is the master pro-inflammatory pathway, while transforming growth factor ? (TGF?) signaling is a major anti-inflammation pathway. Balance between pro- and anti-inflammation signaling networks of TLR and TGF? defines the immune responses. In sepsis and vascular inflammation, TGF?-elicited inhibition of TLR signaling occurs through the induction of inhibitory Smad6. In periodontal tissues, we found that Smad6 is highly expressed in gingival epithelium and disturbance of Smad6 activity exacerbated periodontal bone loss, suggesting that TGF?-elicited inhibition of TLR signaling may regulate periodontitis through controlling gingival epithelial signaling network. We investigated mechanisms of Smad6 action in gingival epithelial cells and identified a novel step in signaling cascades for TLR inhibition that involves Smad6 methylation. We hypothesize that Smad6 methylation suppresses inflammation in gingival epithelium to maintain homeostasis and ameliorate periodontitis through switching on TGF?-elicited inhibition of TLR signaling. We propose to use mouse genetic models (Aim 1 and 2) to investigate roles of Smad6 methylation in periodontal homeostasis and disease, and cell and biochemical approaches (Aim 3) to understand the molecular regulatory mechanisms of periodontal inflammatory responses. This novel mechanism involving Smad6 methylation is a previously unknown step in TLR signaling regulation and introduces new point of intervention with therapeutic potential.